Explaining LiAlH₄, NaBH₄ Reactivities

Lithium aluminum hydride and sodium borohydride are two common hydride reducing agents, and organic chemists have known that LiAlH₄ is more reactive than NaBH₄. The origin of this difference has not been thoroughly explored, however. Joseph S. Francisco, Paul G. Wenthold, and colleagues at Purdue University believe they have an explanation based on a new study of the thermodynamic properties and bonding of dialane (J. Am. Chem. Soc. 2005, 127, 11684). Dialane, Al₂H₆, is structurally similar to diborane, B₂H₆, but its much less stable and hasnt been well-characterized. The researchers find that the gas-phase hydride affinity of AlH₃ to form AlH₄^–— the energy cost of a hydride reduction—is the same as that for BH₃ to form BH₄^–. This means the two anions should have the same reducing capabilities in the gas phase. The researchers conclude that the difference in reactivities of LiAlH₄ and NaBH₄ in solution is probably related to the cations and/or solvent properties rather than to the properties of the hydride anions.